This invention relates to an improved method of making or modifying molecular sieves and molecular sieve-based catalysts. More particularly, this invention relates to an improved method for the nondestructive ion exchange of molecular sieves using an ion exchange solution comprising a trivalent cation, a trivalent cation complexing agent, and a sufficient amount of a hydroxide-producing component to form an ion exchange solution having a pH ranging from about 4 to about 8.
Natural and synthetic crystalline molecular sieves are generally useful as catalysts and adsorbents. These molecular sieves have distinct crystal structures which are demonstrated by X-ray diffraction. The crystal structure defines cavities and pores which are characteristic of the different species. The adsorptive and catalytic properties of each molecular sieve are determined in part by the dimensions of its pores and cavities. Thus the utility of a particular molecular sieve for a particular application depends at least partly on its crystal structure. Because of their unique molecular sieving characteristics, as well as their catalytic properties, crystalline molecular sieves are particularly useful in such applications as gas drying, physical separation, and hydrocarbon conversion processes. Although many different molecular sieves and methods of making and modifying these sieves have been disclosed in the prior art, there continues to be a need for still better and improved molecular sieves and methods for making and modifying such molecular sieves.
Synthetic molecular sieves are often prepared from mixtures containing alkali metal hydroxides and therefore, can have alkali metal contents of 1 percent by weight or more. The ion exchange of various metals or ammonium ion for such alkali metals is generally performed in catalysis in order to obtain active sites that will facilitate particular catalytic reactions. For example, ion exchange of other metal cations or ammonium ion for such alkali metals can be performed to modify catalyst acidity subject to the particular reactions desired and the feedstock and operating condition constraints inherent to the process for conducting these reactions. In other cases, ion exchange of other metal cations or ammonium ion for such alkali metals can be performed to obtain a particular type of activity or selectivity conductive to catalyzing a particular type or degree of reaction. Typical exchangeable alkali metals include sodium or potassium in the sieve and such an ion exchange can be performed with components such as ammonium nitrate or acetate, followed by a subsequent heating step for releasing ammonia, wherein a proton remains at the exchangeable site. This type of ion exchange generally leaves the molecular sieve in the "hydrogen form."
For purposes of the present invention, the term "ion exchange" shall mean the method of changing one cation for another cation at the ion exchangeable sites in the pores of the molecular sieve. This term does not refer to the elemental replacement of one framework element by another potential framework element. Framework elements are generally those elements that are tetrahedrally bonded through oxygen to each other for providing the typical molecular sieve framework.
Similarly, the term "ion exchangeable sites" shall mean the site(s) in a molecular sieve occupied by the cation that balances the negative charge of the electron rich framework tetrahedra.
Metal cation ion exchanges such as aluminum ion exchange can also be performed and generally involve the addition of a molecular sieve to an ion exchange solution comprising a metal salt such as aluminum nitrate and water as exemplified by the following example:
Below a pH of about 4 EQU Al(NO.sub.3).sub.3.9H.sub.2 O(s).fwdarw.Al(H.sub.2 O).sub.6.sup.+3 +3NO.sub.3.sup.-1 (sol)+3H.sub.2 O EQU Al(H.sub.2 O).sub.6.sup.+3 .fwdarw.Al(OH)(H.sub.2 O).sub.5.sup.+2 +H.sup.+ EQU NaMS(s)+Al.sup.+3 (sol).fwdarw.Al MS(s)+3Na.sup.+1 (sol)
where MS is a molecular sieve, Na and Al are the sodium and aluminum ions respectively, and where (s) and (sol) designate the solid species and species dissolved in solution respectively.
Trivalent cation ion exchange can be particularly beneficial, compared to divalent and monovalent cation ion exchange, due to advantages in molecular sieve stability and enhanced activity and selectivity.
However, it is generally known in the prior art that trivalent cation ion exchanges with aluminum can be very difficult to effect. See Carvajal, Chu, and Lunsford, The Role of Polyvalent Cations in Developing Strong Acidity: A Study of Lanthanum-Exchanged Zeolites, Journal of Catalysis 125, 123-131 (1990). In Dealumination of Large Crystals of Zeolite ZSM-5 by Various Methods by Kornatowski, Rozwadowski, Schmitz, and Cichowlas, J. Chem. Soc., Faraday Trans., 88(9), 1339-43, it is noted that ion exchange of ZSM-5 with Al.sup.+3 by using aqueous solutions of Al salts is impossible. The salts of metallic cations, and particularly the trivalent cations such as aluminum, generally form acidic solutions when dissolved in water. For example, the pH of aluminum nitrate generally ranges from about 1 to about 3. Maintaining a low trivalent cation ion exchange solution pH is generally necessary to keep the aluminum in solution. Where the pH of the ion exchange solution comprising an aluminum trivalent cation is increased beyond a level of about 4, the lower solution acidity creates a competition between the aluminum ion exchange reaction and hydroxide ion wherein the aluminum cation can form the colloidal hydroxide and precipitate from the ion exchange solution according to the following reactions:
Above a pH of about 4 EQU Al(NO.sub.3).sub.3.9H.sub.2 O(s).fwdarw.Al(H.sub.2 O).sub.6.sup.+3 +3NO.sub.3.sup.-1 (sol)+3H.sub.2 O EQU Al(H.sub.2 O).sub.6.sup.+3 .fwdarw.Al(OH).sub.3 (s)+3H.sub.2 O+3H.sup.+
It is generally the formation of the aluminum hydroxide precipitate that will not allow aluminum ion exchange to occur above a pH of about 4. Therefore, practicality has historically dictated that such molecular sieve ion exchanges using trivalent cations be conducted at a pH of below 4.
However, molecular sieve ion exchange using ion exchange solutions having a pH of less than 4 can cause the framework aluminum of a zeolite to be acidically leached from the silicon framework. For non-zeolitic molecular sieves such as a borosilicate or a gallosilicate, the framework boron or gallium can similarly be acidically leached from the silicon framework. Since this leaching effect generally reduces the number of exchangeable sites in the molecular sieve, the level of possible trivalent cation ion exchange is also reduced. This leaching effect generally results in a less acidic sieve and can be undesirable to the catalyst manufacturer or commercial user of the catalyst. With some zeolites, a pH of less than 4 can cause the general collapse of the framework and result in an amorphous material.
Therefore, there is a great need in catalysis, for a method for trivalent cation ion exchange of molecular sieves that avoids the problems inherent to the methods of the prior art and does not acidically leach framework metals from the molecular sieve.
Conventional methods for the ion exchange of molecular sieves, and in particular, the zeolites are disclosed in Zeolite Molecular Sieves, Donald W. Breck, John Wiley & Sons at pages 529-580 (1974).
Ion-exchange of cations into zeolite, and particularly the Y zeolite, has also been studied extensively, including work by H. S. Sherry in J. Phys. Chem. (1968) 72, 4086 and in Advan. Chem. Ser. (1971) 101, 350.
Lactic acid has been used in catalysis for templating zeolite synthesis. For example, U.S. Pat. No. 4,511,547 to Iwayama et al. and U.S. Pat. No. 4,581,216 to Iwayama et al. disclose the use of lactic acid for the formation of zeolites where the cation-lactate is a space-filling material around which the zeolite is crystallized. The lactic acid in the Iwayama et al. references is provided for templating the zeolite during formation and is not used for zeolite modification or for the ion exchange of the zeolite subsequent to formation.
It has now been found that trivalent cation ion exchange of molecular sieves can be performed while minimizing the adverse effects of framework metal leaching by complexing the trivalent cation in a manner so as to keep it from precipitating from the ion exchange solution when the pH of the solution is increased.
It has also been found that increasing the pH of the ion exchange solution to a level ranging from about 4 to about 8, in accordance with the method of the present invention, results in effective ion exchange while substantially minimizing leaching of the framework metal and the reduction in ion exchangeable sites caused by such leaching.
It has also been found that when using the particular complexing solution of the present invention comprising one or more of the alpha, beta, and gamma hydroxy- and amino-carboxylic acids and some crown ethers as exemplified by lactic acid, tartaric acid, glycine, and 15-crown-5, and equivalents thereof, the complexed trivalent cation can generally continue to enter the pores of the molecular sieve and gain access to the exchangeable sites.
It has also been found that when using the particular complexing solution of the present invention comprising one or more of the alpha, beta, and gamma hydroxy- and amino-carboxylic acids and some crown ethers as exemplified by lactic acid, tartaric acid, glycine, and 15-crown-5, and equivalents thereof, the complexed trivalent cation can be effectively released to the molecular sieve exchangeable sites.
It is therefore an object of the present invention to provide a method for the effective trivalent ion exchange of molecular sieves.
It is another object of the present invention to provide a method for effective trivalent ion exchange of molecular sieves at a pH above 4.
It is another object of the present invention to provide a method for trivalent ion exchange of molecular sieves which reduces the level of acidic leaching of framework metals over prior art methods.
Other objects appear herein.